Woodward HighPROTEC - Page 830

Protective Elements
All impedances we have calculated or got from generator / transformer data
sheets are per unit values (pu). However, all impedance setting values used by
the device should be apparent impedance values in Ohm based on the relay
secondary rated voltage and currents. This means that we need to convert all
impedance (pu) values into impedance Ohm values.
We have to take the voltage and current transformer ratios into account. For the generator VT and CT in our
example, we have:
[8.]:
VT
Ratio prim/ sec
=166.67
[9.]:
CT
Ratio prim / sec
=3600
[10.] The general conversion formula is:
Z
secondary
[Ω]=Z
primary
[Ω ]⋅
CT
Ratio prim / sec
VT
Ratio prim/ sec
[11.] For the generator distance protection, all impedance-related setting values have to be converted based on the
generator base impedance Z
B,primary
, which can be calculated as primary Ohm value with the generator power
rating S
G
and the generator rated voltage V
G
as follows:
Z
B , primary
[Ω]=
V
G
2
S
G
=
(20 kV )
2
492 MVA
=0.813 Ω
[12.] The generator primary base impedance Z
B,primary
can be converted to the generator secondary base
impedance Z
B,secondary
using [10.]:
Z
B , secondary
[Ω]=Z
B , primary
[Ω]⋅
CT
Ratio prim / sec
VT
Ratio prim / sec
=0.813 Ω⋅
3600
166.67
=17.56 Ω
Settings for the Phase Distance Protection Z[1]
For the example described above we set one distance protection element Z[1] as local backup for transformer and
high-voltage bus protection, and another element Z[2] as remote backup by relay failure in case of system phase
faults. We go for an MHO characteristic, assuming that an MHO characteristic is also used for the line distance
protection. The impedance reach and time delay settings needs to be coordinated with the transmission system
primary and backup protection and CB failure protection to allow selectivity.
It is supposed that the generator and the transformer are protected via differential protection as primary protection,
to detect faults inside the protected objects. A distance protection as local backup protection for the transformer and
high-voltage bus provides additional backup protection in case that the primary protections fails to trip.
830 MCDGV4 DOK-HB-MCDGV4-2E